Pursuant to the recently released Data-Over-Cable Service Interface Specification (DOCSIS) 3.1, the currently-employed 6 MHz spaced channels will be replaced by channels that range in bandwidth from 24 MHz to 192 MHz. The current modulation format (quadrature amplitude modulation, or QAM) will concurrently be replaced by orthogonal frequency division multiplexing, or OFDM. Most leakage detection equipment currently being used relies on the signature of a 6 (or 8) MHz QAM carrier or the insertion of test carriers between adjacent 6 (or 8) MHz QAM carriers. When OFDM carriers are adopted by the cable companies, these leakage strategies will need to adapt to operate in the DOCSIS 3.1 environment. The Data-Over-Cable Service Interface Specification DOCSIS 3.1, which issued on Oct. 29, 2013, is expressly incorporated herein by reference.
DOCSIS 3.1 introduces the concept of a subcarrier, which is a small slice of the complete channel. The channel can be from 24 MHz to 192 MHz in bandwidth and is allowed to operate in either 4K mode (4096 sub carriers) or 8K mode (8192 sub carriers). In 4K mode, the subcarriers are spaced 50 kHz apart, while in 8K mode the sub carriers are spaced 25 kHz apart. Referring now to FIG. 1, four subcarriers 10, 12, 14, 16 of a DOCSIS 3.1 channel 18 are illustrated. Each of the sub-carriers overlaps with an adjacent sub-carrier. For example, sub-carrier 10 overlaps with sub-carrier 12, while subcarrier 12 overlaps with sub-carrier 14, and so forth. The overlap between the subcarriers creates difficulties in identifying and isolating subcarriers on a spectrum analyzer.
Each DOCSIS 3.1 channel also has a continuous pilot carrier, which occurs at the same subcarrier location in every symbol and are needed for receiver synchronization. The pilot carriers are boosted by 6 dB over all other subcarriers in the OFDM signal or channel. This boosting enables easier location and detection of the continuous pilots.
Each DOCSIS 3.1 channel further includes what is called a cyclic prefix, which is a repeated slice or snapshot of a portion of the DOCSIS 3.1 signal. Each snapshot is taken at the end of a symbol and prepended to the beginning of the symbol. For example, if the cyclic prefix is 5 μ-seconds, then the last 5 μ-seconds of an OFDM symbol is copied and prepended to the beginning of that same OFDM symbol. The resulting symbol is now 5 μ-seconds longer. This process is repeated for every symbol, so that each symbol is prepended by a copy of the last part of that symbol. In the DOCSIS 3.1 environment, the cyclic prefix enables a receiver to overcome the effects of intersymbol-interference and intercarrier interference caused by microreflections in the channel. There are five possible values for the length of the cyclic prefix, and the choice depends on the delay spread of the channel—a longer delay spread requires a longer cyclic prefix.